This invention relates to an improved process for the production of 1,3-propanediol by catalytic hydrogenation of 3-hydroxypropanal (HPA).
1,3-Propanediol is used as a monomer unit for polyesters and polyurethanes and as a starting material for synthesizing cyclic compounds.
Various processes are known for the production of 1,3-propanediol via 3-hydroxypropanal (HPA) which start either from C2 and C1 structural units or from a C3 structural unit, such as, for example, acrolein. When acrolein is used, it is first hydrated in aqueous phase in the presence of an acidic catalyst to form HPA. After removing the unreacted acrolein, the aqueous reaction mixture formed during hydration still contains, in addition to 85 wt % based on total organics of 3-hydroxypropanal, approximately 8 wt % 4-oxaheptane-1,7-dial and further organic components in smaller proportions by weight. This reaction mixture is hydrogenated in the presence of hydrogenation catalysts to produce 1,3-propanediol. The 1,3-propanediol is recovered from the reaction mixture by distillation and/or extraction based methods known to those skilled in the art.
U.S. Pat. No. 5,334,778 discloses a two stage process for hydrogenating 3-hydroxypropanal which yields 1,3-propanediol having a residual carbonyl content, expressed as propanal, of below 500 ppm. The hydrogenation is carried out at 30xc2x0 C. to 80xc2x0 C. to a 3-hydroxypropanal conversion of 50 to 95% and then is continued at 100xc2x0 C. to 180xc2x0 C. to a 3-hydroxypropanal conversion of substantially 100%. Suitable hydrogenation catalysts therein include Raney nickel suspension catalysts, and supported catalysts based on platinum or ruthenium on activated carbon, Al203, SiO2, or TiO2 as well as nickel on oxide- or silicate-containing supports.
According to U.S. Pat. No. 5,015,789, very active nickel catalysts exhibit inadequate long-term stability, with a rapid drop in hydrogenation conversion and reaction speed upon repeated use of the catalyst. This results in frequent replacement of the entire catalyst packing, which is associated with known problems in the disposal and working up of compounds containing nickel. In addition, soluble nickel compounds can form in the process and are released into the product stream, requiring further steps to separate the resulting contaminants.
Copending patent application, Ser. No. 60/099,235 discloses a two-stage process for the production of 1,3-propanediol which comprises hydrogenating an aqueous solution of 3-hydroxypropanal using an oxide-supported metal hydrogenation catalyst in a first, low temperature, stage and continuing hydrogenation in a second, high temperature, stage using an activated carbon-supported metal hydrogenation catalyst.
Hydrogenation processes may be characterized by the conversions, selectivities, and space-time yields achievable therewith. Percent conversion of 3-hydroxypropanal is defined by:   X  =            %      ⁢              xe2x80x83            ⁢      Conversion      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      HPA        =                            moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          HPA          ⁢                      xe2x80x83                    ⁢          converted                          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          HPA          ⁢                      xe2x80x83                    ⁢          supplied                    xc3x97      100      
Selectivity of the hydrogenation process is a measure of the amount of converted 3-hydroxypropanal which is converted into the desired product:       %    ⁢          xe2x80x83        ⁢    Selectivity    =                              moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          1                ,                  3          ⁢                      -                    ⁢          propanediol                            moles        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        HPA        ⁢                  xe2x80x83                ⁢        converted              xc3x97    100  
The space-time yield is another important characteristic for continuous hydrogenation processes, stating the achievable quantity of product per unit time and reaction volume.
When hydrogenating 3-hydroxypropanal to 1,3-propanediol on a large industrial scale, it is vital, with regard to the economic viability of the hydrogenation process and the quality of the product, for conversion and selectivity to be as close as possible to 100%. The 1,3-propanediol may be separated from the water as well as remaining 3-hydroxypropanal and secondary products contained in the product stream by distillation after the hydrogenation. However, this distillative separation is rendered very difficult by residual 3-hydroxypropanal and secondary products and may even become impossible due to reactions between the residual 3-hydroxypropanal and 1,3-propanediol to yield acetals such as 2-(2xe2x80x2-hydroxyethyl)-1,3-dioxane (HED), which have a boiling point close to the boiling point of 1,3-propanediol. Thus, the lower the conversion and selectivity, the poorer the achievable product quality.
In order to produce 1,3-propanediol economically, it is also important for the catalyst to exhibit high activity for the hydrogenation of 3-hydroxypropanal. The objective should thus be to find a process in which the smallest possible quantity of catalyst is necessary for the production of 1,3-propanediol; i.e., it is desirable to achieve the greatest possible conversion of 3-hydroxypropanal to 1,3-propanediol with a small volume of catalyst.
Another important quality criterion for hydrogenation catalysts is their operational service life. Good catalysts should ensure high conversion and selectivity in the hydrogenation of 3-hydroxypropanal to 1,3-propanediol over the course of their service life.
It is an object of the present invention to provide an improved process for the preparation of 1,3-propanediol via the hydrogenation of 3-hydroxypropanal whereby the service life of the hydrogenation catalyst is extended.
The present invention provides an improved process for the hydrogenation of 3-hydroxypropanal to 1,3-propanediol which comprises purifying an aqueous solution of 3-hydroxypropanal by contacting said aqueous solution with a purifying agent prior to hydrogenation.
The present invention further provides a process for the preparation of 1,3-propanediol which comprises: a) contacting an aqueous solution of 3-hydroxypropanal with a purifying agent; and b) hydrogenating said aqueous solution of 3-hydroxypropanal to 1,3-propanediol.
The process of the current invention comprises an improved process for the hydrogenation of 3-hydroxypropanal (HPA). In the first stage, an aqueous HPA solution is contacted with a purifying agent; in the subsequent stage (or multiple stages), the contacted HPA solution is hydrogenated to 1,3-propanediol (PDO).
There are various hydrogenation processes known for the conversion of HPA to PDO that utilize aqueous HPA feeds. The purification stage of the present invention is applicable to, and combinable with, all said processes. Most preferably, the purification stage of the present invention is applied to the hydrogenation processes disclosed in U.S. Pat. No. 5,334,778 and in copending patent application, Ser. No. 60/099,235.
It has been found that hydrogenation catalysts are deactivated more rapidly than desired in the preparation of PDO via the hydrogenation of HPA. The deactivation of the catalysts is believed to be due to the adverse interaction between impurities in the aqueous HPA feed and the hydrogenation catalysts employed.
It has been found that treatment of the aqueous HPA feed with a purifying agent prior to hydrogenation improves performance and lifetime of the hydrogenation catalysts. Surprisingly, the initial activity of the hydrogenation catalyst is particularly enhanced. Purifying agents useful in the process of the present invention comprise purifying carbons, purifying silica compositions, diatomaceous earths and zeolites.
The most preferred purifying agents useful in the process of the present invention comprise purifying carbons.
When the purifying agents employed in the process of the present invention are purifying carbons, the carbons are those typically employed as decolorizing carbons. Typically they are activated carbons commercially available from various vendors.
After the purifying agent contacting stage of the process is carried out, the hydrogenation may be carried out using the methods disclosed in U.S. Pat. No. 5,334,778, incorporated herein by reference. For example, stirred reactors or flow reactors can be used. A fixed-bed hydrogenation reactor is particularly suitable for conducting the hydrogenation on an industrial scale. In such a reactor, the liquid reaction mixture flows or trickles over the fixed-bed catalyst together with the hydrogen introduced. To ensure good distribution of the hydrogen in the reaction mixture and uniform distribution of the gas/liquid mixture over the entire cross-section of the fixed bed, the liquid reaction mixture and hydrogen can be passed together through static mixers before the catalyst bed. Trickle bed reactors are particularly preferred and are described in Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Volume 19, pages 880-914 (especially page 884).
The 3-hydroxypropanal is generally fed to the reactor as an aqueous solution having a 3-hydroxypropanal concentration of between 2 and 20 wt %, preferably between 5 and 15 wt %, based on the weight of water and feed, and a pH between about 2.5 and 7.0, preferably between about 3.5 and 5.5. In continuous processes, liquid hourly space velocities between about 0.1 and 10 per hour are preferred. The hydrogenation reaction is conducted at a hydrogen pressure of about 5 to 300 bar, preferably at a hydrogen pressure of less than about 90 bar, more preferably from about 10 bars to 60 bars.